The present invention is generally directed to a circuit for determining light power. More specifically, the invention is directed to a circuit for determining light power received by a photodiode of an optical receiver wherein the bias voltage of the photodiode is controlled by a controller depending on the received light power.
In an article by E. Braun and B. Stummer, "Digitalsignaluebertragung auf Lichtwellenleitern im Fernnetz", Telcom report 10 (1987) Special [Issue] "Multiplex- und Leitungseinrichtungen", Pages 94 through 98, there is disclosed equipment for digital signal transmission via light waveguide cables that contain line terminating equipment and intermediate regenerators comprising optical transmitters and optical receivers. Due to attenuation of light signals in the waveguide cables, it is desirable to measure optical reception power in such optical communication transmission systems, particularly in those having great dynamics.
In an optical receiver including an avalanche photo diode (APD) that contains a control means for controlling the bias voltage of the APD, the bias voltage of the APD can be utilized for displaying the received light power. However, since the characteristic bias voltage curve varies greatly in view of the slope, it must be linearized by segment formation. Further, the characteristic curve has an extremely steep slope given high diode multiplication factors. As a result, it is nearly impossible to obtain sufficiently precise readings of the received light power for low light powers. Further, the evaluation of this characteristic curve is not suitable for the indication of the "no light" condition without great complications.